In Australia, the electric resistance welding (ERW) method is the principal method for linepipe manufacture. ERW requires steel strip feed that has been sourced from a hot strip mill. ERW involves heating side edges of strip to temperatures above the melting point of the steel and thereafter butting and thereby welding the side edges together.
Mn levels in linepipe steels produced by the applicant have historically been in the range of 0.8-1.5 wt. %. The main purpose of the Mn addition is to provide solid solution strengthening.
However, one disadvantage of Mn is that it has strong segregating tendencies that typically manifest in anomalous microstructures (with high hardness and low toughness) at the centreline of strip produced by hot rolling continuously cast steel slabs.
Such anomalous microstructures can have deleterious effects on the mechanical properties of the ERW weld line particularly when linepipes are produced from centre slit strip feed. Specifically, the step of butting side edges of strip together in ERW diverts Mn rich centreline segregation bands at the slit side edge of strip into the plane of the weld that forms. The result is to compromise the toughness of the weld line.
Such anomalous microstructures arising from centreline segregation can also have deleterious effects on welds produced by welding together aligned ends of linepipes during construction of a line using upset welding processes such as MIAB and flash butt welding. These processes involve induction heating the rims of the aligned ends by and then butting the ends together to produce autogenous welds. Currently the MIAB welding process, which has some operational and economical advantages over conventionally used manual welding of line pipe, is not used widely in pipe line construction.
The adverse effect of centreline segregation is exaggerated by the coincident occurrence of elongated MnS inclusions in the strip. The plasticity of MnS inclusions in the hot rolling process increases directly with increasing Mn level. The detrimental effects of MnS inclusions on ductile fracture propagation resistance of pipeline steels are well known. These inclusions exert a controlling influence on the fracture toughness of both the pipe body and the weld line. The adverse effect on the weld line toughness is particularly evident for the case of pipes made from centre slit strips.
Traditionally, the adverse effects of elongated MnS inclusions and centreline segregation have been controlled by limiting S concentrations in steel to be below 0.005% (or even lower limits) depending on the specific requirements of a pipeline.
In addition, some steel manufacturers, as an additional countermeasure, have the capability to achieve complete sulphide shape control using Ca injection processes.
However, there are significant capital and operating costs associated with both of the above measures, and the measures are not attractive options for these reasons.